1. Field of the Invention
The present invention concerns a process for preparing solutions containing the electrolytes used in lithium-polymer batteries of the ACEP type (polymer electrolyte battery). More specifically, the invention is concerned with a process for transferring the polymer directly from the solvent of the polymer synthesis, to the more polar solvent of the polymer-salt complex. Generally, the invention aims at the formation of a ternary solvent mixture including the solvent of the polymer synthesis, the solvent of the polymer-salt complex and a non-solvent of the polymer, and at the achievement of two phases where the polymer is found in the lower phase containing most of the solvent of the polymer-salt complex.
2. Description of Prior Art
Lithium-polymer electrolyte ACEP batteries are manufactured by superposing three main types of films: a film of a positive electrode containing an electrochemically active material, for example vanadium oxide, a film of an electrolyte consisting of a polymer and a lithium salt, and a film of metallic lithium. Each of these films is between 15 and 50 .mu.m thick, for a total thickness of the elementary film of battery of 100 to 150 .mu.m. About 30 meters of this elementary film 15 cm wide, are typically required to give a battery of 100 Wh.
The families of polymers used in the electrolyte films are described in Armand's U.S. Pat. No. 4,303,748, as well as in more detail in U.S. Pat. Nos. 4,578,326 and 4,357,401, and Canadian Patent No. 1,269,702. For example, they may consist of amorphous copolymers and terpolymers, which may be cross-linkable. These polymers are not presently commercially available, and are mainly synthesized with coordination catalysts of the type described in Vandenberg U.S. Pat. No. 3,205,183 (September 1965). This polymerization may only be carried out in relatively non polar solvents such as ethyl ether or toluene.
The polymer-lithium salt mixtures used in the electrolytes and positive electrodes are usually not soluble in these solvents, and more polar ones such as those based on acetonitrile, must be used to solvent-cast the films, a commonly used method for the electrolytes, and the only one for positive electrodes.
To switch from the synthesis solution containing only the polymer to the electrolyte solution containing the polymer-lithium salt mixture, the solvents must therefore be changed. One way to proceed consists in precipitating the polymer from the synthesis solution by means of a non-solvent, in drying the precipitate then in re-dissolving the latter in the new solvent, with the lithium salt. The last two steps are long and costly, particularly with polymers of high molecular weights (Mw&gt;100,000).